The traveling wave tube is an electron tube used for the amplification of an RF (Radio Frequency) signal, the oscillation, or the like by the interaction between an electron beam emitted from an electron gun and a high-frequency circuit. For example, as shown in FIG. 6, a traveling wave tube 1 includes an electron gun 10, a helix 20, a collector 30, and an anode 40. The electron gun 10 emits electrons. The helix 20 is a high-frequency circuit in which an electron beam 50 formed of electrons emitted from the electron gun 10 interacts with the RF signal. The collector 30 captures the electron beam 50 outputted from the helix 20. The anode 40 leads out electrons from the electron gun 10 and guides the electrons emitted from the electron gun 10 inside the helix 20 that is spiral-shaped.
The electron gun 10 includes a cathode 11 which emits electrons (thermal electrons), a heater 12 which gives heat energy for emitting the electrons (thermal electrons) to the cathode 11, and a wehnelt 13 which forms the electron beam 50 by focusing the electrons emitted from the cathode 11. For example, the cathode 11 is made with a disc-shaped cathode pellet consisting of a porous tungsten base which is impregnated with an oxide (an emitter material) such as barium (Ba) or the like. For example, an electron gun (a pierced electron gun) equipped with the wehnelt 13 is described in patent literature 1 (PTL1) and the like.
The electrons emitted from the electron gun 10 are accelerated by the electric potential difference between the cathode 11 and the anode 40 while forming the electron beam 50 and guide into the helical structure of the helix 20. The electrons guided into the helical structure of the helix 20 travel through the helical structure of the helix 20 while the introduced electrons interact with an RF signal inputted from one end of the helix 20. The electron beam 50 which passes out through the helical structure of the helix 20 is captured by the collector 30. At this time, the RF signal amplified by the interaction with the electron beam 50 is outputted from the other end of the helix 20.
In the electron beam 50, because the electrons with a negative charge are repelled from each other by the coulomb force, diameter of the electron beam 50 is increased according to the travel distance of the electron. Accordingly, a periodic magnetic field generation device (not shown) which generates the magnetic field for suppressing the expansion of the electron beam 50 passing through the helical structure of the helix 20 is disposed in the periphery of the helix 20 and the diameter of the electron beam 50 is kept constant over the whole length of the helix 20 by the magnetic field generated by the periodic magnetic field generation device. The periodic magnetic field generation device is described in, for example, patent literature 2 (PTL2).
Further, in patent literatures 3 and 4 (PTL3 and PTL4), it is described that the electron beam can be controlled by the magnetic field. In PTL3, it is described that magnetic field applying means such as a coil or the like is used for deflecting the electron beam. Further, in PTL4, it is described a structure in which in order to prevent the electron gun from being magnetized and thereby keep the trajectory of the electron beam stable, magnetic erasing means composed of a coil is disposed in the periphery of the electron gun.
As shown in FIG. 6, a negative direct-current high voltage (body voltage Ebody) determined by using an electric potential HELIX of the helix 20 as a reference is supplied to both the cathode 11 and the wehnelt 13 from a power supply device (not shown). A positive or negative direct-current voltage (in FIG. 6, a negative voltage: a heater voltage Ef) determined by using an electric potential H/K of the cathode 11 as a reference is supplied to the heater 12. A positive direct-current high voltage (an anode voltage Ea) determined by using the electric potential H/K of the cathode 11 as a reference is supplied to the anode 40. Further, a positive direct-current high voltage (a collector voltage Ecol) determined by using the electric potential H/K of the cathode 11 as a reference is supplied to the collector 30. Usually, the helix 20 is connected to a case (a body) of the traveling wave tube 1 and grounded.
FIG. 6 shows an example of a structure of the traveling wave tube 1 including one collector 30. However, the traveling wave tube 1 may have a structure in which a plurality of the collectors 30 are included. Further, FIG. 6 shows an example in which the anode voltage Ea is supplied to the anode 40. However, the traveling wave tube 1 may be used in a state in which the anode 40 is grounded. Further, FIG. 6 shows an example in which the wehnelt 13 is connected to the cathode 11. However, the traveling wave tube 1 may have a structure in which a positive or negative direct-current voltage (a wehnelt voltage Ew) determined by using the electric potential of the cathode 11 as a reference is supplied to the wehnelt 13.
In the traveling wave tube 1 shown in FIG. 6, an amount of the electrons emitted from the cathode 11 can be controlled by the anode voltage Ea and the electric power of the RF signal outputted from the traveling wave tube 1 can be controlled by the anode voltage Ea. The similar control can be performed by the wehnelt voltage Ew applied to the wehnelt 13. Further, an amount of the electrons which can be emitted from the cathode 11 depends on the temperature of the cathode 11, in other words, the temperature of the heater 12. Therefore, in the traveling wave tube 1, the heater voltage Ef is set according to the output power of the RF signal.
For example, in patent literature 5 (PTL5), it is described a structure in which the electric power of the RF signal outputted from the traveling wave tube 1 is controlled by the anode voltage Ea. In PTL5, it is described that the output power of the RF signal is controlled by the anode voltage Ea and the heater voltage Ef is adjusted according to the output power of the RF signal.